--> --> Sequence stratigraphy of the Mississippian Antler foreland basin

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Sequence stratigraphy of the Mississippian Antler foreland basin

Abstract

The quality of the oil shale source rocks or sweet spots within the Mississippian Antler foreland basin (AFB) is dependent upon the stratigraphic sequence. Sequences of the AFB are best preserved in the deeper-water proto-Oquirrh basin portion in central Utah where sequences were not removed by erosion. Nearer the Antler Orogeny in central Nevada much of the rock record was removed during periods of relative sea level lowering. Classic sequence boundary root zones of Stigmaria mark the base of regressive sequence boundaries. The richest source rocks with the highest organic content typically occur in lacustrine shales that lie above the regressive sequence boundary unconformity. Siliciclastics usually coarsen and thicken upward in the sequence. Transgressive sequence boundaries generally contain fragments from the top of the underlying regressive sequence and separate non-marine rocks from marine rocks. Except for some marginal marine shale, transgressive rocks are generally lean in organic content. Subsequent compression during the Late Cretaceous and Early Tertiary western American tectonic event shortened the basin sequences by folding and thrust duplexes. Thicknesses of Antler clastics can change from more than ten thousand feet in a very short distance. Not only does the thickness change between thrust sheets but also organic richness and maturation of the shale changes dramatically. The best oil shows and production so far occurs near the leading edge of a thrust fault that likely has fifty miles of displacement. Restorable balanced structural cross sections help restore the thrust sheet to its original position and provide evidence of large swaths of land where buried source rocks could have charge overlying structures.